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Abstract:

The present invention relates to a glass melting furnace, comprising a
channel-shaped melting tank, the batch material being charged at an
upstream end, the molten glass being recovered at the downstream end,
said furnace being heated by means of burners, in which 80% of the
combustion energy is produced by oxycombustion, oxygen being supplied
continuously from a production plant located nearby or via a gas pipe
from remotely located plants, characterized in that the furnace is fitted
with oxygen storage means such that, should continuous supply cease, the
furnace can operate at least in a temperature-maintaining mode for a
maximum time of eight hours.

Claims:

1. A glass melting furnace, comprising:a channel-shaped melting tank,
wherein the introduction of batch materials is conducted at an upstream
end, and molten glass is recovered at the downstream end;at least one
burner which heats said furnace, in which, with the at least one burner,
at least 80% of the combustion energy is produced by oxy-combustion,
wherein oxygen is supplied continuously from a production plant located
nearby or via a gas pipe from a remotely located plant; andan oxygen
storage unit, which, should continuous supply cease, assures operation of
the furnace at least in a temperature maintaining mode for a minimum
period of eight hours.

2. The melting furnace according to claim 1, wherein maintenance of the
temperature is assured by adding at least one additional burner that
operate by air combustion, andwherein a proportion of energy of the at
least one additional burner does not represent more than a third of total
energy in the temperature maintaining mode.

3. The furnace according to claim 2, comprisinga traditional melting
zone,a traditional refining zone, anda traditional conditioning zone,
wherein since the traditional conditioning zone does not have a
significant supply of energy in ordinary operation, the at least one
additional burner operating by air combustion is located in the
conditioning zone.

4. The furnace according to claim 2, wherein the air for the air
combustion is preheated in heat exchangers.

5. The furnace according to claim 4, comprising:a fuel supply comprising
either liquid fuel oil or gas; andat least one heat exchanger which
preheats the gas,wherein the at least one exchanger optionally serves to
heat the air during the temperature maintaining mode, and a fuel is
liquid fuel oil.

6. The furnace according to claim 1, wherein power delivered by the at
least one burner in the temperature maintaining mode is at most equal to
a third of power in ordinary operation.

7. The furnace according to claim 6, wherein, in the temperature
maintaining mode, only some of the burners for oxy-combustion are kept in
operation.

8. The furnace according to claim 7, wherein the burners kept operative
are located so that fumes sweep over practically an entire surface of a
bath before being evacuated.

9. The furnace according to claim 1, wherein in an ordinary operating mode
and in the temperature maintaining mode, at least 65% of fumes are
evacuated upstream of the furnace in the vicinity of a location of
charging of batch materials.

10. A process for maintaining a temperature of the furnace according to
claim 1, the process comprising keeping the burners active such that
crown temperatures in melting and refining zones are not less than
1100.degree. C. and in a conditioning zone are not less than 1050.degree.
C.

11. The process according to claim 10, wherein elements in contact with
molten material that are normally cooled, are either removed from a bath
(blenders) or their cooling is interrupted (dams).

12. The process according to claim 10, wherein when the furnace comprises
bubblers, operation thereof is maintained strictly at a minimum so that
the bubblers are not blocked by setting of the glass.

13. The furnace according to claim 1, wherein the oxygen storage unit
assures operation of the furnace at least in the temperature maintaining
mode for a minimum period of at least 20 hours.

14. The furnace according to claim 1, wherein the oxygen storage unit
assures operation of the furnace at least in the temperature maintaining
mode for a minimum period of at least 30 hours.

15. The furnace according to claim 6, wherein, in the temperature
maintaining mode, at most half of the burners for oxy-combustion are kept
in operation.

16. The furnace according to claim 3, wherein the air for the air
combustion is preheated in heat exchangers.

17. The furnace according to claim 16, comprising:a fuel supply comprising
either liquid fuel oil or gas; andat least one heat exchanger which
preheats the gas,wherein the at least one exchanger optionally serves to
heat the air during the temperature maintaining mode, and a fuel is
liquid fuel oil.

18. The furnace according to claim 2, wherein power delivered by the at
least one burner in the temperature maintaining mode is at most equal to
a third of power in ordinary operation.

19. The furnace according to claim 3, wherein power delivered by the at
least one burner in the temperature maintaining mode is at most equal to
a third of power in ordinary operation.

20. The furnace according to claim 2, wherein power delivered by the at
least one burner in the temperature maintaining mode is at most equal to
a third of power in ordinary operation.

Description:

[0001]The present invention relates to glass melting furnaces in which the
melting energy is essentially produced by burners supplied with fuel and
oxygen or a gas that is very rich in oxygen. These furnaces are usually
referred to as "oxy-combustion" furnaces.

[0002]The secondary use of oxy-combustion burners is well known in glass
melting furnaces. Thus, it is a matter of adding one or a limited number
of oxy-combustion burners to furnaces operating with air in a traditional
manner. The introduction of these additional burners is generally
intended to increase the capacity of existing furnaces, possibly as their
performance declines because of their age. It can also be to simply
increase the capacity of a given furnace by introducing additional
sources of energy.

[0003]The addition of some oxy-combustion burners to large-capacity
furnaces is usually performed without any significant modification in the
general operation of the furnace. In particular, the proportion of energy
discharged from oxy-combustion remains low. It does not exceed 20% and
very frequently does not exceed 10% of the total. For this reason, the
continued supply of oxygen does not pose any significant risks to the
plant. If the supply of oxygen is interrupted for whatever reason, the
temperature of the furnace can be maintained by means of the only burners
operating with air combustion. The output is possibly modified
momentarily, but the plant is not put in danger.

[0004]Apart from having a supplementary source of energy, the systems
operating in the so-called oxy-boosting mode are not able to benefit from
all the known advantages that can result from oxy-combustion. A lower
energy consumption and reduced emissions of undesirable fumes are among
the potential advantages.

[0005]Moreover, the reduction of energy consumption per production unit in
question has the advantage of restricting the carbon dioxide emissions
and therefore meeting statutory requirements in this field.

[0006]The presence of nitrogen is also a source of the formation of
so-called NOx oxides, the emission of which is forbidden in practice
because of damage associated with the presence of these compounds in the
atmosphere. The use of oxygen enables the problems associated with
nitrogen in the air to be excluded, which is not the case with the
oxy-boosting techniques.

[0007]Despite the advantages outlined above, the use of oxy-combustion in
large glassmaking furnaces is yet to be developed. There are various
reasons for this. Thus, the use of oxygen is necessarily more costly than
that of air. However, the economic evaluation of using oxy-combustion is
positive, particularly when a significant portion of the heat from the
fumes is recovered as described in the unpublished European Patent
Application 08 102 880.5 filed on 25 Mar. 2008. Moreover, continuity of
the operation requires supplying the oxygen without interruption.

[0008]In the case of large-capacity furnaces the necessary volume of
oxygen is such that the most reliable supply is either by gas pipe or by
a production plant in situ. Whatever mode is adopted, one problem is to
ensure that the supply is permanent with consideration that a momentary
interruption is always a possibility. However, in these circumstances the
conditions that do not pose a risk to the plant must be maintained.

[0009]The invention relates to the methods for implementing the
oxy-combustion technique in large glassmaking furnaces that form the
subject of the claims attached to this description.

[0010]The first objective in the case of interruption to supply is to have
means that allow the plant to be kept at temperature levels at which the
molten glass remains in this state. For this, taking into consideration
the significant and inevitable energy loss, a certain energy supply must
be maintained in the furnace. Given that the plant is designed to operate
using oxy-combustion, it is necessary to be able to supply energy
according to this method using present means. The burners for
oxy-combustion cannot be used like those for operation with air. The
respective volumes of fuel are not in the same order of magnitude. The
circuit and the pipe systems used with oxygen cannot be used for
circulating air. For this reason, the thought is not to replace
oxy-combustion with air combustion, except possibly in the case indicated
further below for some burners located in the refining zone.

[0011]Therefore, to be able to supply the burners it is necessary to have
an emergency source of oxygen available. Nevertheless, the difficulty
lies in having a sufficient reserve available to cope with the momentary
absence of supply. Considering the consumption in normal operating mode,
the emergency stored volumes cannot be sufficient to assure full
operation of the installation. According to the invention, to maintain
these stores at a level where the cost is not prohibitive, the operation
of the furnace is placed in slowed-down mode in the case of momentary
stoppage of oxygen supply. The supply of energy is reduced strictly to a
minimum to keep the glass molten. The furnace is no longer supplied with
batch material and potential cooling sources are kept as far away as
possible.

[0012]Given the type of assumed risk and commitments made by the oxygen
suppliers, it must be possible to maintain the temperature of the furnace
for a period of at least 8 hours. The quantity of oxygen stored on site
at the plant must be sufficient to maintain this status at least for this
period. Nevertheless, where possible, a higher quantity is preferably
stored to better exclude any risk of supply failure that would exceed
that which the suppliers presume to guarantee. It is advantageous to
store a quantity of oxygen that corresponds to 20 hours of operation and
preferably corresponds to 30 hours of operation.

[0013]The reduced supply of energy is intended to keep the glass present
in the furnace in molten state, but is not intended to melt materials
once again. In these conditions, the amount of energy necessary can be
reduced significantly. Advantageously, the energy necessary does not
represent more than a third of the energy consumed during the normal
operating mode, with consideration of the inevitable losses.

[0014]In the temperature maintaining mode, the equilibrium of the furnace
is greatly modified. The energy supply is distributed in a different
manner to that during normal operation.

[0015]The molten material is distributed in different zones of the
furnace: the melting, refining and conditioning zones. The supply of
energy during normal operation is localised in the first two zones. The
conditioning zone, which precedes the float bath, is intended to reduce
the temperature of the glass to progressively bring it to sufficient
viscosity. The absence of heating means above the conditioning zone does
not allow the temperature to be maintained. To prevent the glass from
setting, therefore, it is necessary to provide means to compensate for
the absence of renewal of the molten glass material in this zone, said
renewal alone assuring that the temperature is maintained in normal
operating mode.

[0016]According to the invention, it is advantageous to provide emergency
burners above the conditioning zone. These burners can be permanently
installed or can be removable, in this case only being provided in case
of need. The removable burners are advantageously arranged in the glory
holes, which in normal mode allow evacuation of the hot atmosphere above
the conditioning zone, thus promoting the desired cooling.

[0017]If these burners are removable, their mode of supply must be suited
to this special feature. Because they must be put into operation quickly,
they must have supply pipes that are easily connected. The connection to
the supply circuit for fuel, whether liquid or gaseous, does not pose any
particular difficulties. Conversely, this movable characteristic makes a
supply of oxygen, particularly hot oxygen, a very delicate matter. For
this reason, the emergency burners used above the conditioning zone are
preferably burners that operate with air combustion. It must be noted
that the benefit of using oxy-combustion is necessarily set aside in
these emergency procedures. The presence of an air combustion section is
not for the purpose of optimising the energy supply, but simply to
maintain the temperature in the furnace.

[0018]The operation of the burners is arranged for an optimised system.
Each burner operates in a power range that can vary within certain limits
around a nominal reference power. To retain a good energy efficiency, it
is not desirable to deviate too far from this range. Therefore, as a
result of the reduction in energy supply, it is necessary to limit the
number of active burners in the temperature maintaining mode.

[0019]The choice of burners that are kept active takes into consideration
the specificities of oxy-combustion, but also of this particular mode of
operation which no longer requires the melting of batch materials.

[0020]The oxy-combustion system, as described in the European Patent
Application cited above, comprises a specific circulation of fumes inside
the furnace. This circulation for the largest portion of the fumes is
directed upstream of the furnace to benefit the transfer of energy to the
zones of the furnace that require the largest supply.

[0021]Beyond the circulation of fumes, the majority of these collect for
the purpose of recovering as much of the energy they carry as possible.
This recovered energy is used in particular for preheating the oxygen
and, if necessary, the fuels consumed.

[0022]The furnaces in question must also be substantially devoid of an
atmosphere charged with nitrogen. For this reason, all the burners of the
furnace mostly operate by means of oxy-combustion. However, while it is
possible to retain a portion of the air combustion, the energy generated
by oxy-combustion represents at least 80% of the total energy used in the
furnace, and preferably at least 90% thereof.

[0023]Regardless of the constituents of the atmosphere of the furnace
coming from the combustion, it is still necessary to prevent as far as
possible the entry of external air in order to prevent an energy loss
corresponding to the reheating of this air, but above all to prevent as
far as possible the formation of undesirable NOx due to the passage of
this air through the high temperatures of the flames (these temperatures
are in the order of 1800° to 2300° C., depending on the
type of oxygen burner chosen).

[0024]Irrespective of the envisaged designs, glassmaking furnaces cannot
be kept perfectly sealed against the outside atmosphere. Moreover, to
prevent the admission of ambient atmosphere, the circulation of fumes in
the furnace is arranged so that it develops a dynamic seal.

[0025]It is not possible to fully modify the fittings of the furnace to
take into account the specific conditions of the emergency mode. However,
it is possible to select burners that are kept operative and those that
are momentarily stopped.

[0026]In this particular system, the fumes continue to be directed
upstream of the furnace but this is only to continue maintaining the
energy recovery for heating the oxygen used. In normal operating mode, a
limited portion of the fumes is extracted in the downstream section of
the refining zone so that the restricted volume of air coming from the
conditioning zone is evacuated with these fumes. The risk of nitrogen
oxides forming is thus avoided. The purpose of maintaining the current of
air coming from the conditioning zone is to prevent any entrainment of
suspended dust in the atmosphere of the furnace.

[0027]In the emergency operating mode, the main concern is to maintain the
temperatures. A momentary introduction of a little air into the furnace
is tolerable. In these conditions, it is preferable according to the
invention to ensure that all the fumes are evacuated upstream. On the one
hand, the use of energy is best transmitted to the molten material, and
on the other hand, even in the emergency mode when less fumes are
discharged, the recovery of the heat therefrom is maintained at as high a
level as possible, assuring conditions that allow the heating of the
oxygen used in particular are maintained.

[0028]Since the supply of batch materials is interrupted, the amount of
energy necessary in the upstream section of the furnace is much reduced.
Even if the energy of the burners is reduced as indicated above, thermal
equilibrium can be reached in this upstream section even if the first
burners are not kept operative.

[0029]The distribution of the power by the choice of burners kept
operative tends to make the temperatures of the melting and refining
zones uniform. This distribution is also achieved in order to maintain as
far as possible convection loops inside the bath. These flows, even when
slowed down, continue to assure a reheating of the sections most exposed
to cooling. This concerns the floor of the furnace in particular.

[0030]In general, to achieve this result the majority of the energy is
supplied by the burners located in the central zone at the boundary of
the melting and refining zones. Upstream, as indicated, the need for
energy is lower than in normal operation. In contrast, in the absence of
any flow of molten material towards the refining zone, it may be
necessary in this zone, as well as in the conditioning zone, to provide
burners in addition to those operating in normal operation.

[0031]Advantageously, if additional burners are provided in the refining
zone, these burners also operate with oxygen to retain the benefit of
oxy-combustion as far as possible. In the case where these burners are
not installed permanently, the removable supply circuits are badly suited
to the use of hot oxygen. The operation of these additional mobile
burners in the refining zone is therefore preferably assured with oxygen
that is not heated.

[0032]The choice of oxygen burners is all the more preferred as it assures
that the temperatures are appropriately maintained in this zone of the
furnace. Additional burners operating with air combustion that would be
arranged to face existing glory holes would not enable the required
temperatures to be appropriately maintained. The space requirement of the
usual mobile burners means that these would cause the flame to develop in
the thickness of the walls of the furnace. The heating of the crown in
these conditions would only be assured by contact with the combustion
gases. Their temperature is not sufficiently high to keep the crown at
the desired values.

[0033]The arrangement of the air combustion burners, the flames of which
would develop from the walls, is possible in principle. However, such an
arrangement is not usual. It would require the removal of refractory
elements from the walls of the furnace, an operation that is particularly
inconvenient. Arrangement at the time of construction of emergency
burners is also possible, but is not preferred for reasons of
practicality.

[0034]While the burners intended for oxy-combustion are not usable like
those for air combustion for the reasons indicated above, modifications
to these burners allow an alternative operation, if necessary. It has
been mentioned that the replacement of oxygen by air is not possible
using the supply pipes of these burners. An alternative solution for
these burners is to maintain the introduction of fuel and that of the
so-called "primary" oxygen introduced with the fuel. This primary oxygen
assures permanent ignition. The supplies of "secondary" and possibly
"tertiary" oxygen, which represents the largest portion of the fuel and
assures stepped combustion in the flame, are interrupted. Thus, instead
air is introduced through appropriate conduits arranged in refractory
blocks adjacent to the "primary" flame. The flames in these conditions
develop well in the chamber of the furnace. Consequently, their radiation
allows the crown to be maintained at adequate temperatures. However, the
placement of these air supply pipes is a relatively difficult operation.
For this reason, it is only undertaken if it is not possible to maintain
the alternative modes described above.

[0035]In normal operating mode, the temperatures of the crowns of the
furnace amount to between 1350° and 1450° C. Reducing the
energy supply and maintaining the bath in molten state are accompanied by
an appreciable decrease in temperature of these crowns. However, these
temperatures remain elevated. In the melting and refining zones the
temperature must preferably not be less than 1100° C. In the
conditioning zone the temperature in the normal operating mode is
controlled to enable glass at about 1100° C. to be available when
it is poured into the float bath. To reach these temperatures, not only
are there no burners provided, but significant ventilation is maintained
in order to progressively lower the temperature of the glass. In these
conditions the temperature of the crown is appreciably lower than that in
the melting and refining zones.

[0036]As indicated above, in the emergency mode it is necessary to supply
energy to prevent the glass from setting. However, the crown temperature
can be a little lower than that in the preceding zones. It must
preferably not fall below 1050° C.

[0037]Convection loops are maintained in the normal mode of operation not
only as a result of natural mechanisms caused by differences in the
temperatures existing between the zones of the furnace, but also as a
result of movements caused by supplementary means. These are, for
example, bubblers, blenders or dams. What all these have in common,
besides benefiting these convection movements, is to cause some cooling
of the glass.

[0038]The blenders or dams, which are exposed to very aggressive
conditions, must be permanently cooled during use. The objective is to
minimise losses in the temperature maintaining mode. On this basis, the
blenders are advantageously removed from the bath or their cooling is
interrupted. Cooling is also interrupted for the dams, which are
obviously not removable.

[0039]It is difficult to completely break off the supply of the bubblers
arranged on the floor. Some flow of gas is maintained because these must
be prevented from becoming blocked as a result of the glass setting. To
limit the cooling associated with this introduction, the flow is reduced
strictly to a minimum.

[0040]In practice, in the case of the oxy-combustion furnaces in question,
the use of means for heating the bath by immersed electrodes is not
envisaged, or if so to a very limited degree, e.g. in the charging zone
for batch materials. In all cases the energy supply in normal operating
mode does not exceed 5% of the total. These means are quite obviously
independent of the supply of oxygen and for this reason can contribute to
the energy supply for operation of the emergency mode.

[0041]To obtain an advantageous economic balance of large oxy-combustion
furnaces, it is necessary to recover a significant portion of the heat
contained in the fumes discharging from the furnace. In practice, as in
the case of air combustion furnaces, the most economic use of this
recovered energy lies in reheating the reagents introduced into the
furnace: oxygen, fuel and possibly batch materials.

[0042]Heating the oxygen requires very strict precautions. The
installations in which the oxygen circulates must be perfectly sealed,
resistant to elevated temperatures and to the oxygen brought to these
temperatures. The preheating of the oxygen is advantageously conducted in
exchangers made from steels that have an excellent resistance to hot
oxygen. Exchangers and materials suited to this use are described in the
unpublished European Patent Application No. 07 107 942 filed on 10 May
2007. To minimise the risks associated with the movement of hot oxygen,
various arrangements are proposed in the already cited European Patent
Application No. 08 102 880.5.

[0043]In substance, two types of arrangements are proposed. Firstly, the
transfer of energy is conducted in two stages. In a first stage, the
fumes are passed into a recuperator and the transfer of energy occurs
with an intermediate heat transfer fluid, e.g. air. In a second stage,
the heat transfer fluid passes into an exchanger where it reheats the
oxygen. Secondly, the exchangers heating the oxygen are located as close
as possible to the burners to reduce the risks associated with corrosion.
Seams and connections are avoided as far as possible in the same manner.

[0044]This system of recovery continues to operate in the reduced mode.
The treatment is simply dedicated to heating the oxygen supplying the
burners that are kept active.

[0045]In practice, to guarantee perfect operating reliability, it is
preferred that the temperature of the oxygen is maintained at less then
650° C., preferably less than 600° C.

[0046]The fuel used is advantageously preheated whether it is natural gas
or liquid fuel oil. In the case of natural gases, the preheating
temperature is advantageously less than 650° C. and preferably
less than 550° C. In the case of heavy fuel oils, the temperature
is generally less elevated and does not exceed 180° C., and
preferably does not exceed 150° C., to prevent the fouling that
results from cracking of these fuels.

[0047]The invention is described in some detail below with reference to
the sets of drawings:

[0048]FIG. 1 is a schematic perspective view of a furnace according to the
invention;

[0049]FIG. 2 is a schematic plan view of the arrangements of FIG. 1 in
emergency operating mode.

[0050]The furnace shown in FIG. 1 is of the type used in large-scale glass
production operations such as those used to supply flat glass production
using float glass techniques. Furnaces of this type continuously produce
quantities of glass that can amount to as much as 1000 tonnes/day. To
achieve these outputs the furnaces must have available power up to 60 MW.

[0051]The furnace 1 comprises a tank disposed in a closed chamber. The
assembly is made from refractory materials resistant to temperatures, the
corrosion of the fumes and the aggressive action of the molten materials.
The bath level in the tank is shown by a broken line 2.

[0052]The furnace is supplied with batch materials at one of its ends. The
opening, via which these batch materials are charged is shown
schematically at 3. In practice, to facilitate distribution over the
surface of the bath several charging points are usually provided. The
molten glass exits at the opposite end via a neck 4 with a reduced width
in relation to that of the tank. The base of the neck 4 is usually at the
level of the floor of the furnace.

[0053]The neck is not completely immersed in the molten glass. There
remains a space between the upper part of the neck and the surface of the
strip of glass. The operating conditions relating to the gas flows in the
furnace are regulated so that the atmosphere of the furnace does not pass
into the neck in order to prevent any risk of suspended dust particles
being entrained. To assure this operation, it is preferable to preserve a
weak current of gas circulating contrary to the flow of the molten glass.
Since the intention is only to prevent a current of gas in the reverse
direction, this current is kept as weak as possible.

[0054]Burners, the placement of which is indicated at 6, are arranged
along the side walls of the furnace on each side thereof to spread the
flames over practically the entire width of the tank. The burners are
spaced from one another in order to distribute the energy supply over a
large portion of the length of this same melting and refining tank.

[0055]The combustion gases F are evacuated for the most part through the
outlets 7 located close to the charging zone and at some distance from
the closest burners. In the drawings (FIGS. 1 and 2), two outlets 7 are
arranged symmetrically on the side walls, while the charging of batch
materials (MP) is in the axis of the furnace. This is a preferred
embodiment, but other arrangements are also possible such as e.g. the
outlet of the gases in the wall 8 closing the furnace in its upstream
portion. These outlets can also be distributed differently, but it is
important to ensure that the fumes rise in counterflow to the flow of the
glass V in the furnace. If necessary, the fumes can be at least partially
output through the charging opening or openings.

[0056]As indicated above, according to the invention it must be ensured
that the chamber of the furnace is virtually sealed against the
penetration of outside air. The circulation of the fumes upstream
prevents penetration on this side of the furnace. Any passages arranged
in the side walls are also substantially sealed against the penetration
of ambient air. In order to force back the small amount of air that can
arrive in the conditioning section 5, a very restricted circulation of
the fumes downstream of the furnace is advantageously arranged. These
fumes F' are evacuated through the outlets 9.

[0057]The three zones of the furnace are indicated as I (melting), II
(refining), III (conditioning) in FIG. 2.

[0058]The boundary between melting and refining is not generally embodied
in the structure of the furnace. In particular, if a dam is arranged on
the floor in these furnaces, this dam does not normally coincide with
this boundary, even if it has a part in the conditions that lead to its
positioning.

[0059]The distinction between the melting and refining zone is functional
in all cases. It corresponds to the mode of circulation of the glass in
the tank. This corresponds to a first convection loop in the melting
section and a loop that rotates in the reverse direction to the first in
the refining section. In the absence of any means directly influencing
the circulation, the position of the boundary of the melting zone and the
refining zone is determined by a collection of operating parameters that
includes in particular the distribution of energy by the burners.

[0060]As a general rule, during normal operation, the supply of energy
necessary for melting the batch materials is more significant than that
which maintains the glass at temperature for refining. Consequently, the
number of burners, and above all the power they deliver, is more
significant in the melting zone.

[0061]According to the operating modes that prove most advantageous, the
"heat curve", i.e. the distribution of the temperatures along the
furnace, firstly progresses from upstream up to a central section close
to the start of the refining zone. The temperature then varies a little
decreasing slightly towards the neck 4 in preparation of the passage into
the conditioning zone. For this reason, the downstream end of the furnace
is normally devoid of burners.

[0062]The distribution of the burners is shown in FIG. 2 by the axis
thereof. They are preferably arranged in staggered rows on either side of
the tank to ensure that the flames emitted in opposite directions do not
strike one another.

[0063]The conditioning zone 5 does not contain any burner in the normal
operating mode. Openings 11 are arranged in the walls and ambient air is
sent into the chamber 5 to bring the glass to a temperature compatible
with its application on the float bath.

[0064]The operation in the temperature maintaining mode is illustrated in
FIG. 2. Only one section of the burners is kept operative. In the example
shown, the burners being numbered according to their position along the
furnace starting from the charging zone, only burners 3, 8 and 9 of the
ten burners, which are the burners permanently installed in the furnace,
are active. The active burners are illustrated by the schematic
representation of the corresponding flames.

[0065]The downstream end of the refining zone does not normally contain
active burners. The flow of molten glass is sufficient to maintain the
temperature conditions. In the emergency operating mode, the flow of
glass is stopped. The end of the refining section therefore requires a
supply of energy. In the embodiment shown, two additional burners a and b
are temporarily arranged at locations prepared in advance in the walls of
the furnace. Since the burners operate in the chamber that is changed to
oxy-combustion, it is preferable to ensure that these burners are
oxy-type burners. Their detachable nature makes the supply of hot oxygen
very difficult. To assure all the required safety, the construction of
the pipe systems and exchangers used for the movement of hot oxygen
requires special features that are not very compatible with mobility. For
these reasons, the oxy-burners a and b are preferably supplied with cold
oxygen. However, in view of the temporary nature of the operation of
these burners, it is possible to provide air burners. Thus, some of the
advantages of oxy-combustion are suspended momentarily. In contrast, the
use of air burners enables the consumption of oxygen to be reduced and,
in the case of a given emergency reserve, thus enables the availability
of this reserve to be extended.

[0066]In normal operating mode, the conditioning zone 5 does not contain
any device for heating the glass. In the emergency mode it becomes
necessary to supply the minimum amount of energy to keep the glass in
molten state. Since no fumes are generated in the conditioning chamber,
this likewise has no conduit for conveying these fumes out of this
chamber. In these conditions, it is advantageous to choose air burners
for the installed burners c and d. Once again a step is taken to avoid
taking from the oxygen reserve, which is a reserve of necessarily limited
volume. Moreover, the use of removable burners means that the positioning
of the flame does not allow the crown to be heated to higher
temperatures. However, in the refining section the temperature required
is lower than in the melting and refining section. The use of these
burners allows these temperatures to be reached.

[0067]The fumes F at the outlet of the furnace are used in devices
intended for the recovery of a portion of the energy entrained by these
fumes. Advantageously, this recovery serves to heat the oxygen of the
burners. The most reliable system for heating oxygen comprises a double
system of thermal exchange with an intermediate heat transfer fluid. The
system in question described in the patent application cited above
comprises a first exchange in a recuperator, the heat transfer fluid
heated in this recuperator is then passed into an exchanger for heating
the oxygen there and this is then conveyed to the burners.

[0068]Upon exit, the fumes are initially at temperatures in the order of
1200° to 1400° C. It is preferable to direct them into a
recuperator, in other words a basic exchanger, which enables the
temperature of the fumes to be lowered for their treatment before being
discharged into the atmosphere. The heat transfer fluid, e.g. air, can be
brought to a very high temperature, e.g. in the order of 800° C.
This air is directed towards the exchangers to heat the oxygen. The
temperature of the oxygen at the outlet of these exchangers can be as
high as 600° C., but preferably does not exceed 550° C. The
exchangers heating the oxygen are preferably in the immediate vicinity of
the burners to minimise the path of the hot oxygen to its point of use.

[0069]In the emergency mode of operation, the circulation of the fumes
remains essentially the same as in the normal mode. The fumes, while less
abundant than in the normal mode, can again serve to heat a likewise
reduced quantity of oxygen.

[0070]As an indication, in a furnace such as that shown in FIG. 1, the
power supplied to obtain an output of 600 tonnes/day of glass is 60 MW.
In the emergency mode of operation, the power supplied by burners 3, 8
and 9 is not more than about 7 MW. The two additional burners a and b in
the refining zone add about 2.5 MW and the burners c and d located in the
conditioning zone for their part supply 4 MW. In total the power is 13.5
MW, a little less than quarter of the operating power in the production
mode. To supply the burners in oxy-combustion producing 9.5 MW, the
oxygen necessary is in the order of 2000 Nm3/h. A reserve of liquid
oxygen of 80 000 litres allows the furnace to be supplied for a period of
as much as 30 hours, which is in principle much higher than that
anticipated for the supplier to re-establish a sufficient supply.
Otherwise, a supply via mobile tank must necessarily predominate.